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An abnormally high number of eosinophils in the blood. Normally, eosinophils constitute 1 to 3% of the peripheral blood leukocytes, at a count of 350 to 650 per cubic millimeter. Eosinophilia can be categorized as mild (less than 1500 eosinophils per cubic millimeter), moderate (1500 to 5000 per cubic millimeter), or severe (more than 5000 per cubic millimeter).

In areas of the world where parasitic diseases are common, they are the usual cause of eosinophilia. In developed nations, eosinophilia is most often due to allergy or, less often, a drug reaction. There are numerous other causes of eosinophilia, but individually they are quite uncommon.

Eosinophilia may be primary or secondary. In primary eosinophilia, the increased production of eosinophils is due to an abnormality in a hematopoietic stem cell as, for example, in eosinophilic leukemia. In secondary eosinophilia, the increased production of eosinophils is a reactive process driven by cytokines, as is the case in allergy.